1. Field of the Invention
This invention relates to the field of programmable fuses, and particularly to methods for reading the state of such fuses.
2. Description of the Related Art
Programmable fuses are commonly used as non-volatile memory devices, with a blown fuse representing, for example, a logic ‘1’, and an intact or unblown fuse representing a logic ‘0’. A programmable fuse in combination with the components required to read its state (i.e., intact or blown) is referred to as a “fuse cell”.
Numerous techniques have been developed to determine whether a particular fuse is blown or intact. One common method is to apply a known current through the fuse. The voltage developed across the fuse due to the fuse's resistance (Rfuse) is compared with a predetermined threshold value to determine whether the fuse represents a ‘1’ or a ‘0’. However, when replicated across a large number of fuses, current consumption for this approach can be unacceptably high.
Another fuse cell reading method is illustrated in FIG. 1. Here, a programmed fuse 10 is installed in the output leg of an upper current mirror 12 made from FETs MP1 and MP2, and a reference resistance Rref is installed in the output leg of a lower current mirror 14 made from FETs MN1 and MN2. The current mirrors are connected at a node 16, which is buffered with a logic gate 18 that produces an output OUT.
Resistance Rref is selected to establish a threshold with which Rfuse is compared to determine whether fuse 10 is intact or blown. In operation, supply voltage VDD is applied to the cell, output OUT is latched, and the cell is powered down. Assuming MP1 and MN1 are matched and that MP2 and MN2 are matched, node 16 is pulled down (and OUT goes high and is latched) when Rfuse>Rref, and node 16 is pulled up (and OUT goes low and is latched) when Rfuse<Rref.
However, when arranged as shown, the voltage across fuse 10 is relatively small. This renders the cell particular sensitive to mismatches between the Vgs voltages of the cell's FETs. Increasing this voltage requires an undesirable increase in current.